When I
decided to start transmitting on 10MHz I had to face the "antenna dilemma". I
did not want to buy a ready made antenna, I wanted to homebrew it. Thus I had to
cope with the size of my house's roof. A vertical was out of question, due to
thelack of suitable supports. An
inverted V dipole was the
only antenna I could install...But the roof is too small for a full size dipole,
even in an inverted V configuration. I decided to use RG58 coaxial cable to take
advantage of the velocity factor of the cable. Thus the length of each arm of
the dipole was reduced to 4,88 metres instead of 7,39 metres. Because I had two
pieces of cable of just 3.50 metres I decided to set up a dipole and test it in
the garden. As expected it was too short and resonated somewhere around 14MHz. I
than started adding pieces of ordinary electric wire to the coax moving the
resonant frequency downward up to 10MHz..."et voila" the antenna was ready! The
final tuning was done with the antenna on the roof.

Fig. 1 is a
sketch of the antenna: the coaxial cable to the rig is connected to the inner
conductor of the coax of the dipole and these pieces are shortened (braid twisted
to the inner conductor) at their end where the single wire is soldered.

Fig. 3 shows
a very interesting characteristic of this particular design: the fact that
the antenna has3 resonant frequencies! 10MHz, 16MHz and
36MHz. And that these 3 frequencies are not harmonically related. They depend on
the length of the segments the antenna is made with. 10MHz is the total length
of the antenna (coax+single wire), 36MHz is the single wire
portion. Up to now I cannot
figure where the 16MHz comes from!

Fig. 3 the 3 resonant
frequencies

Although
this antenna works quite well I don't think it is worth to be copied as I built
it with cables I found in the junk box! For the next one I build I will properly
cut the coax to resonate the antenna without the use of "tails"...